1. Field of Invention
The invention relates to probes comprising peptide nucleic acids (PNAs), and to a method of using PNA probes to sequence or to assay nucleotides in solution, without solid support or adjacent double stranded nucleotide construct.
2. Description of Related Art
PNAs are polyamide analogs of DNA and RNA. See, e.g., U.S. Pat. No. 5,539,082 to Nielsen et al. Nielsen et al. discloses that PNAs mimic natural polynucleotides by binding complementary single stranded (ss) DNA and RNA strands. PNAs generally comprise ligands linked to a peptide backbone. Representative ligands include either the four main naturally occurring DNA bases (i.e., thymine, cytosine, adenine or guanine) or other naturally occurring nucleobases (e.g., inosine, uracil, 5-methylcytosine or thiouracil) or artificial bases (e.g., bromothymine, azaadenines or azaguanines, etc.) attached to a peptide backbone through a suitable linker.
Probes comprising PNA sequences have been employed to detect target nucleotide sequences. U.S. Pat. No. 5,503,980 to Cantor suggests employing PNA probes in a method of sequencing a nucleic acid by hybridizing the nucleic acid with a set of PNA probes containing random, but determinable, base sequences within the single stranded portion adjacent to a double stranded portion, wherein the single stranded portion of the set preferably comprises every possible combination of sequences over a predetermined range. Hybridization occurs by complementary recognition of the single stranded portion of a target with the single stranded portion of the probe and is thermodynamically favored by the presence of adjacent double strandedness of the probe.
However, although Cantor discloses that the nucleic acids can be PNAs, it does not disclose or suggest utilizing such probes in the absence of a solid support. Moreover, the present invention does not require the adjacent construct of DNA material being tested.
In addition to teaching the use of a solid support like Cantor, Perry-O'Keefe et al., "Peptide Nucleic Acid Pre-Gel Hybridization: An Alternative to Southern Hybridization," 93 Proc. Natl. Acad. Sci. USA 14670 (December 1996) also teaches that PNA does not generally bind well to double stranded DNA (dsDNA). See Perry-O'Keefe et al. at page 14673, footnote. Moreover, the homopyrimidine PNA constructs which have been found to bind dsDNA well would not be useful as probes. Applicants have discovered that the qualification which suggests that only homopyrimidine can bind with dsDNA by strand inversion is incorrect and arises from the hybridization conditions employed.
Smulevitch et al., "Enhancement of Strand Inversion by Oligonucleotides Through Manipulation of Backbone Charge," 14 Nature Biotechnology 1700 (Dec. 1996) (disclosed in Landsdorp, "Close Encounters of the PNA Kind," 14 Nature Biotechnology 1653 (Dec. 1996)) discloses using PNA primers to hybridize with dsDNA. However, Smulevitch et al. teaches the use of gels in detecting hybridization, and does not suggest the use of fluorescent markers.
Many types of sample analysis rely upon the fluorescent properties of a stain. Fluorescence occurs when a molecule excited by light of one wavelength returns to the unexcited (ground) state by emitting light of a longer wavelength. The exciting and emitted light, being of different wavelengths, can be separated from one another using optical filters, a camera or a CCD. Fluorescence has been used to visualize certain molecules (and hence structures) by light microscopy for many years, and is also used in other analytical techniques, such as flow cytometry. Further, the emission of fluorescence showing different colors can be detected by a human eye, a camera or a charge coupled device (CCD).
For example, U.S. Pat. No. 5,594,138 to Dykstra et al. discloses a method of fluorescent detection of a nucleic acid. The method comprises contacting the nucleic acid with a fluorescent marker that is a bis-dicationic aryl furan compound and exposing the nucleic acid to light at a frequency inducing fluorescence of the fluorescent marker. The fluorescent marker may be conjugated to a nucleotide sequence as a probe for hybridization studies, or it may be conjugated to numerous reagents for in situ labeling studies.
U.S. Pat. No. 4,963,477 to Tchen discloses a probe of high sensitivity containing a modified nucleic acid, which can be recognized by specific antibodies.
Fluorescent In Situ Hybridization (FISH) is a technique comprising detecting fluorescent probe binding to human chromosomes by attaching DNA to a solid support, such as a glass slide. See, e.g., K. H. Andy Choo, Ed., "In Situ Hybridization Protocols," Chapters 2 and 4 (Humana Press, Totowa, N.J., 1994). Like all other conventional detection methods comprising hybridization with probes, this method relies on the solid support to keep the two complementary strands of DNA apart while the probe hybridizes with one of the strands. In addition, FISH requires a complicated buffer and temperature control protocol, with overnight incubation.
Until the present invention, however, it has not been possible to rapidly test for the presence of nucleotide sequences in solution using a method which does not destroy the sample, is less hazardous to laboratory personnel than radiation based assays, does not require the cost and delay of preparing solid supports, and is readily automated. Time and cost efficient detection of mutant genetic sequences has been the rate limiting step in correlating mutant genotypes with altered phenotypes. Although conventional DNA sequencing methods have been considered to be the most accurate means of identifying mutations, these methods have been relatively slow and labor intensive, and are not particularly well-suited to rapidly screening large numbers of samples of genomic DNA.
All references cited herein are incorporated herein by reference in their entireties.